The present description relates to a method of starting an internal combustion engine, more particularly to a method of starting a spark ignition engine without using a starter motor.
In recent years, to improve fuel economy of vehicle engines, particularly for city driving, there has been the development of so called idle stop control. This method automatically stops the vehicle's engine when stop conditions are met, for example when the vehicle is stopping at a traffic light. The engine is automatically restarted when a restart condition is met or upon a restart request, such as when the driver operates the accelerator pedal for the vehicle launch from the traffic light.
A method of the idle stop control is presented such as in European Patent Application publications EP1403511A1 and EP1544456A2. This method does not use a conventional electric starter for automatically restarting the engine because of starter motor durability concerns and because electric power consumption may be excessive due to the potential frequent use of the electric starter during idle stop control. Instead, the method first injects fuel directly in a cylinder, which is on the compression stroke when the engine stops and is referred to as “compression stroke cylinder”. Then, it ignites the mixture of air and fuel in the compression stroke cylinder. As a result, combustion of the ignited mixture generates higher pressure in the compression stroke cylinder and, moves the piston downward. The downward movement of the piston moves the crankshaft in reverse direction for a short interval, because the piston is in the compression stroke and it is supposed to move upward during four cycle engine operation.
The reverse rotation of the crankshaft causes movement of pistons in other cylinders as well. A piston in a cylinder, which is on the expansion stroke when the engine stops and is referred to as “expansion stroke cylinder”, is moved upward by the reverse rotation of the crankshaft. The upward moving piston compresses the air in the expansion stroke cylinder. Then, fuel is directly injected in the expansion stroke cylinder and, the mixture is ignited and combusted to generate a higher pressure in the expansion stroke cylinder. The higher pressure pushes down the piston to move the crankshaft in the forward direction, thereby initiating the forward or normal rotation of the crankshaft. Movement of the piston also causes the other pistons to move because the pistons are linked together through the crankshaft. The piston in the compression stroke cylinder ascends and approaches the compression top dead center (hereafter referred to as “first compression top dead center”). Then, generally, the mixture in the compression stroke cylinder is already combusted or used up and does not make energy to crank the crankshaft. So, a cylinder that makes torque after the expansion stroke cylinder is a cylinder that is on the intake stroke when the engine stops and is therefore referred to as “intake stroke cylinder”.
As the crankshaft continues to rotate, a piston in the intake stroke cylinder now moves into the compression stroke from the intake stroke. The molar mass of air contained in the intake stroke cylinder is close to the molar mass of air that the cylinder contained when the piston passed through bottom-dead-center, cylinder pressure was near intake manifold pressure, and when the cylinder volume was greatest. On the other hand, the compression stroke cylinder and the expansion stroke cylinder contained less air molar mass than some other cylinders, because some air may leak from the cylinder over time during the engine stop due to the pressure difference between the inside and outside of the cylinder. Then, the piston in the intake stroke cylinder compresses the full molar mass of the air and the cylinder pressure therein rises, as the crank shaft rotates forward on the inertia exerted by the combustion in the expansion stroke cylinder. When the piston in the intake stroke cylinder approaches the compression top dead center (hereafter referred to as “second top dead center), the pressure in the intake stroke cylinder might be so high that the piston does not pass the second compression top dead center. If the piston passes the second compression top dead center, the mixture in the intake stroke cylinder may be ignited and combustion may generate enough energy for subsequent continuous rotation of the crankshaft. So, for an engine restart, it is desirable that the rotational inertia on the crankshaft overcomes the counterforce exerted by the pressure in the intake stroke cylinder at the second top dead center.
To increase the inertia of the crankshaft at the second compression top dead center, the EP1403511 publication presents a method of combusting air and fuel mixture in the compression stroke cylinder following the first combustion in the expansion stroke cylinder. Specifically, it leaves some fresh air in the compression stroke cylinder after the combustion for the reverse rotation by setting the initial air fuel ratio lean of the stoichiometry and injects additional fuel afterwards. Then, the mixture is of remaining air and the additional fuel just is ignited just after the first compression top dead center, thereby deriving additional energy to crank the engine from the compression stroke cylinder. Alternatively, the '511 publication presents a method to open the intake valve of the compression stroke cylinder at the late stage of the reverse rotation and close it at the early stage of the forward rotation so that some fresh air is inducted into the compression stroke cylinder. The mixture of newly inducted air and remaining or newly injected fuel in the compression stroke cylinder can be ignited after the top dead center, thereby deriving the additional energy to crank the engine from the compression stroke cylinder to increase the inertia of the crankshaft at the second top dead center.
For the same purpose, the EP1544456A2 publication presents a method of reducing pressure in the compression stroke cylinder at the first compression top dead center to reduce the counterforce acting against the inertia of the crankshaft. Specifically, it injects additional fuel into the compression stroke cylinder after the combustion for the reverse rotation in the compression stroke cylinder so that evaporative latent heat of the additional fuel cools down the combusted gas and decreases the pressure in the compression stroke cylinder. The decrease of the pressure in the compression stroke cylinder leads to a decrease of the counterforce acting against the inertia of the crankshaft.
Although the above prior methods may improve the success rate of the engine starting, the inventors herein have recognized that there is still need to increase the rotational inertia of the crankshaft at the second top dead center for a more reliable engine restart, more specifically there is still room to increase the torque exerted by a first combustion after a restart request.